Gold, silver, and elements of the platinum group are known as valuable precious metals and are used not only as jewelry and currency but also in various industrial applications. The elements of the platinum group include platinum, palladium, rhodium, ruthenium, iridium, osmium, and the like. Rhenium, which is usually not categorized as a precious metal but is an industrially valuable metal as well, is also a scarce and expensive metal.
For example, gold is used in bonding wires for semiconductor integrated circuits and in contact materials for electronic substrates. Silver is used in photography films and conductive paste. Platinum, palladium, and rhodium are used in catalysts, electrode materials, temperature sensors, medical equipment, and electronic materials. Rhenium is used as thermocouple, catalysts, and the like.
Being scarce and expensive, gold, silver, the elements of the platinum group, and rhenium (hereinafter in the present invention, these are collectively called “precious metals”) are obtained by collecting and recycling used products, defective products yielded during manufacturing processes, and the like. Employed as a method of recycling precious metals are a dry process of melting and separating in a furnace at a high temperature and a wet process of melting in acid and the like and then separating by a method such as neutralization, solvent extraction, crystallization, and electrowinning. The dry process has advantages of having excellent productivity to treat a large quantity in a single process and requiring no additional separation step, but it also has a problem in the retrieval rate in retrieving precious metals, namely, loss of precious metals. On the other hand, the wet process has an advantage in terms of its capability of retrieving with little loss of precious metals, but it also has a problem that retrieval of precious metals at high purity is accompanied by challenging separation from other coexisting components. Because of these, retrieval is carried out by considering the advantages of both processes and adopting either the dry process or the wet process, or both of these in combination.
As the wet process, various techniques are developed. For example, Patent Document 1 describes a method of retrieving platinum group metals at high purity from raw material containing the platinum group metals. Specifically, elements of the platinum group are sequentially separated and retrieved according to the following procedure. By the method described in Patent Document 1, the metal Pd having purity of 99.95% can be retrieved at a percent yield of 99%.
(1) A smelting residue is subjected to chlorination with hydrochloric acid and hydrogen peroxide, to give 3 L of a chlorination solution (A). The amount of hydrochloric acid added is determined so as to achieve a concentration of free hydrochloric acid in this solution of 4 mol/L or higher.(2) The solution containing the elements of the platinum group (the chlorination solution A) is mixed with 1 L of DBC (dibutyl carbitol) for 30 minutes to extract gold.(3) Caustic soda is added to the residual solution after gold extraction, followed by neutralization until the concentration of free hydrochloric acid reaches 2 mol/L. The resultant neutralized solution (the residual solution after Au extraction) at a volume of 3 L and 3 L of DHS (dihexyl sulphide) are mixed for 3 hours to extract palladium (Pd).(4) The Pd-containing DHS resulting from the above step (3) is washed with 3 L of hydrochloric acid (concentration: 1 mol/L) and, thereto, 3 L of an aqueous solution of ammonia and ammonium chloride (NH3 concentration: 3 mol/L, NH4Cl concentration: 1 mol/L) is added for back extraction of Pd. To the aqueous solution containing Pd as a result of back extraction, hydrochloric acid is added to achieve a pH of lower than 1, so that Pd yellow is retrieved. The resultant Pd yellow is dissolved in an aqueous ammonia solution, and to the resultant solution, hydrazine is added for reduction.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2012-167367